This invention relates to the art of nuclear reactor power plants and has particular relationship to the transfer of fuel assemblies to and from nuclear reactors during fueling, refueling and the like. While this invention is uniquely applicable to nuclear reactors, it is realized that the invention may have applicability in other arts. To the extent this invention is applied in other arts, such application is regarded as within the scope of equivalents of this invention. It is also understood that while this invention is predominantly practiced to transfer fuel assemblies, it may also be applied to the transfer of individual fuel rods. Such application is within the scope of equivalents of this invention.
During refueling the core of a nuclear reactor with its fuel assemblies is immersed in highly radioactive water in the pressure vessel. The spent fuel assemblies which are to be removed are also highly radioactive. These fuel assemblies are positioned in slots in the core. Typically the spent fuel assemblies are removed to a vehicle which carries them to a pit where they are stored under water as nuclear waste. The replacement fuel assemblies are removed from a vehicle and transferred to the reactor where they are inserted in the slots vacated by the spent fuel assemblies.
The transfer of fuel assemblies between the vehicles and the reactor and the insertion of replacement fuel assemblies in the slots in the core is accomplished by a crane. The crane carries a line which terminates in a fixture for engaging the fuel assemblies. Typically the fixture has fingers which are expansible and collapsible under hydraulic pressure. Each fuel assembly has a hole in its top. The crane has a traverse drive which positions the fixture with the fingers collapsed over a fuel assembly. The line drive then guides the fixture into the hole in the assembly. The fingers are expanded to grip the fuel assembly. Then the line drive is energized to raise and remove the fuel assembly. Other facilities for engaging the fuel assemblies are also feasible. For example, each assembly may be provided in its top with an eyelet to be engaged by a hook terminating the crane line.
In carrying out the transfer of the fuel assemblies, it is necessary that the crane line be guided precisely into the hole in the fixture. It is necessary that the operator know if the fixture has encountered an obstruction at the boundaries of the slot which contains a fuel assembly. Positive intelligence must be available that a spent fuel assembly has been effectively engaged by the crane fixture. If a fuel assembly breaks during removal, a part of the broken radioactive assembly may lodge in the reactor under the water. This event constitutes a major catastrophy as it may disable a reactor altogether. The breaking of an assembly during removal must be avoided. While inserting an assembly in a slot, intelligence that the assembly has encountered an obstruction or the boundary of a slot is essential. It must also be known that the assembly is seated in the slot. There must also be intelligence that a fuel assembly being inserted or removed from a slot has encountered excessive resistance or that the movement of the assembly in or out is obstructed. Such increased resistance, particularly when a fuel assembly is being removed from a slot, may damage the crane by overloading the line drive or may rupture the line. At times it may be necessary to traverse the crane line laterally without raising the line to its uppermost starting position. It is indispensible that such a traverse be carried out with the crane-line and its fixture free of a fuel assembly. Provisions to assure that traverse of the line with a fuel assembly on the fixture is precluded are indispensible.
It is an object of this invention to provide apparatus uniquely suitable for the transfer of fuel assemblies to and from a nuclear reactor in whose use and operation the above-described condition shall be fully met.